As svgmuc says, but change it to something really really secure, letters and digits and no, PA55W0RD is too easy lol. Yahoo allows access to account logins through an API, which makes dictionary hacking so much easier. Yahoo don't seem to monitor failed logins through this route, otherwise this would never happen.

Check also that the hackers have not changed your "Secret Questions". One of my clients got hacked and they changed the first of her secret questions, luckily she had a second question which was left untouched.

Put yourself in your address list so that you are among the first to know that you are in the business of promoting the benefits of viagra(tm).

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If you have a sample of the E-Mail, with complete headers, you can post it here, after obscuring your E-mail address and anything else that tends to personally identify you (and don't bother with the body of the E-Mail, it's the headers that are important).

But short of that, we have no way of knowing which is happening to you.

Ask someone in your address book to forward a copy of the mail headers to you, then post them on here.

This will tell you all kinds of info... what email address the email claimed to be from, the IP address of the computer that sent the mail, the relay it used to get to the recipient, the time it was sent, etc...

Unfortunately there is no easy way to stop this, as some mail servers don't use SPAM filters or blacklists. Traditional email was checked on the basis that all mail was legitimate unless the mail server receiving the mail could provide a valid reason why it should be blocked (using blacklists, spam filters, etc.)

More recently some email providers use technologies such as SPF to make the sender prove that the mail is legitimate.